1. Field of Invention
The present invention relates to a high-pressure discharge lamp light source device and a projector in which the high-pressure discharge lamp light source device is mounted. Particularly, the present invention relates to an alternating current type high-pressure discharge lamp having a light emitting tube in which at least 0.2 mg/mm3 of mercury is sealed, and therefore, the mercury vapor pressure is high (e.g., 110 atm or higher), the high-pressure discharge lamp being suitably used as a projection light source for projection type projector devices, rear projection televisions and the like, a high-pressure discharge lamp light source device comprised of the high-pressure discharge lamp and its light source device, and a projector on which the high-pressure discharge lamp light source device is mounted.
2. Description of Related Art
It is required for a projection type projector device to illuminate images on a rectangular screen uniformly with sufficient color rendition. Accordingly, a metal halide lamp in which mercury or a metal halide is sealed is used as a light source. In addition, the recent trend has been to reduce the size more and more to the level of a point light source, and one having an extremely small distance between the electrodes has practically been used.
Under these circumstances, a high-pressure discharge lamp having an extremely high mercury vapor pressure (e.g., 20 MPa (about 190 atm) or higher) has recently been used in place of a metal halide lamp. The purpose of this lamp is to narrow down the width of the arc by increasing the mercury vapor pressure and further increase the light output. In the above-mentioned ultrahigh-pressure discharge lamp, a pair of electrodes is disposed inside a light emitting tube made of quartz glass in a manner of facing each other with a distance of at most 2 mm, for example, and mercury of not less than 0.20 mg/mm3, a rare gas, and a halogen in the range of 10−6 μmol/mm3 to 102 μmol/mm3 are sealed inside the light emitting tube (See, e.g., Published Unexamined Japanese Patent Application No. H2-148561 and corresponding U.S. Pat. No. 5,109,181).
This type of discharge lamp and its light source device are disclosed in Published Unexamined Japanese Patent Application No. 2001-312997 and corresponding U.S. Pat. No. 6,545,430, for example.
In the high-pressure discharge lamp disclosed in Published Unexamined Japanese Patent Application No. 2001-312997 and corresponding U.S. Pat. No. 6,545,430, the mercury vapor pressure inside the tube is in the range from 15 MPa to 35 MPa during steady lighting state; halogen in the range of 10−6 μmol/mm3 to 102 μmol/mm3 is sealed inside the light emitting tube; a pair of electrodes is provided inside the light emitting tube; and a protrusion is provided in the vicinity of the center of the tip end of the electrode so as to suppress the occurrence of the so-called arc jump phenomenon, the arc jump phenomenon being a phenomenon in which the position of the discharge arc generated between the electrodes becomes unstable and moves around between the center of the tip end of the electrode and the periphery thereof. An alternating voltage is applied between the pair of the electrodes for operation using a power source device comprised of a DC/DC converter, a DC/AC inverter and a high-voltage generator.
On the other hand, as the size of the projector is so reduced recently, so that it can be used in ordinary homes, there is a need for considering a screen that is not too bright in synch with the brightness of the use environment and the type of images to be projected. In response to such a need, a projector having a function referred to as the dimming function has been designed (See, e.g., Published Unexamined Japanese Patent Application No. 2000-131668). Here, the dimming function refers to a function for adjusting the brightness of a lamp and reducing the consumed electric power by operating a high-pressure discharge lamp with electric power lower than the rated electric power. As used herein, the term “dimmed power operation” refers to operation with electric power lower than the rated electric power.
The present high-pressure discharge lamp light source device is usually provided with both the “rated power operation” and the “dimmed power operation.” As used herein, the operation with the “rated power operation” and the “dimmed power operation” is defined as the steady operation. Besides, the “dimmed power operation” is generally operated with electric power that is 60 to 80% of the “rated power operation.”
FIG. 18 shows one example of the electric current waveform when a high-pressure discharge lamp is operated using a light source device having the dimming function. As shown in the figure, the electric power is reduced to about 60 to 80% of the rated power operation for operating a lamp when the dimmed power operation command signal is turned on during the rated power operation.
Furthermore, a projector having a function referred to as the AV mute has recently been designed, for example, that stops projecting images temporarily when the projection of images on a screen is not required.
Such a function can be achieved by mechanically shutting down the discharge lamp considering the fact that relighting is not available immediately after turning off the light of the discharge lamp because the internal pressure is high, by shutting down light to be projected onto a screen by adjusting the voltage to be applied to a liquid crystal panel, or the like. As used herein, the term “standby power operation” refers to the operation of a lamp in a state where images are not projected on a screen deliberately.
In the standby power operation, it is desirable that the power is as low as possible. This is because the operation with power as low as possible allows markedly decreasing heat generated from the lamp and stopping or significantly decreasing the drive of a cooling fan, which is the major cause of noise generated by a projector, thereby making the noise generated by the projector as small as possible.
Moreover, since the thermal load of the lamp can be reduced by operating the lamp with low electric power, the lifespan of the lamp can be prolonged by combining the case in which projection is required with the case in which projection is not required. As used herein, the term “thermal load” refers to the thermal load on the light emitting tube and electrodes, which can be reduced with smaller input power.
Besides, one of the properties of a projector is the contrast ratio. The contrast ratio refers to the ratio between the brightness on a white screen for projection (bright state) and the brightness on a black screen (dark state). When the contrast ratio is high, an image having a clear distinction between brightness and darkness can be projected and, therefore, is an important property of a projector together with the brightness.
In order to increase the contrast ratio, a technology for creating the black state using the above-mentioned shutting down function (iris adjusting function) has recently been adopted.
There is a need for a high-pressure discharge lamp that enables brightness of a prescribed level or higher during the rated power time to be achieved and allows not only for the dimmed power operation as described above, but also the standby power operation, which is achieved by lowering the dimming power as much as possible.
As described above, there is a need for a high-pressure discharge lamp that allows for the standby power operation, which is achieved by lowering the electric power as much as possible. However, the following problems may occur when the power is significantly decreased in a state where rectangular wave alternating current is being supplied.
In this type of high-pressure discharge lamps, as described in Published Unexamined Japanese Patent Application No. 2006-332015, for example, a protrusion is formed on the tip end of the electrode during operation and a stable arc discharge is formed centering on the protrusion. It is described in Japanese Patent Application No. 2006-332015 that the stable operation is possible by maintaining the protrusion, which is the starting point of an arc, wherein the stability can be achieved by changing the frequency or the number of waves of a stationary frequency and a low frequency, which is intermittently (periodically) inserted, depending on lamp voltage or lamp operation power. However, the starting point of the arc became unstable and the so-called flicker phenomenon frequently occurred when a high-pressure discharge lamp (e.g., a lamp with the rated power of 180 W) was operated with 90 W using the abovementioned technology, which problem occurred in any combination of frequencies. Particularly, this problem markedly occurred with 70 W relative to the rated power of 180 W.
In order to solve the abovementioned problem, the object of the present invention is to provide a high-pressure discharge lamp light source device that can positionally stabilize the starting point of an arc in a discharge lamp, thereby preventing the occurrence of the so-called flickers, even when the lamp operation power is extremely low, i.e., the operating power is at most 0.5×P (W) with respect to the maximum operating power P (W) in the steady operation state and that does not influence the lighting operation in the screen projection mode by suppressing the deformation of the electrode as well as a projector on which the high-pressure discharge lamp light source device is mounted.